Method of making arc confining structures



Sept. 19, 1944. G. F. MCMAHON 1 2,358,407

METHOD OF MAKING ARC CONFINING STRUCTURES Original Filed Dec. 9, 1940 2 Sheets-Sheet 1 I I9 I 0 f ORGAN/C h 0? cam/ma [7 INSULAT/fi/V ORGAN/c 0/? CER/I/V/C l/vsuL/lT/o/v ARC EXT/NGU/SH/NG MATERIAL WITH BINDER 0F METHYL CELLULOSE Sept. 19, 1944. v G. F. M MAHON 4 2,353,497

METHOD OF MAKING ARC CONFINING STRUCTURES I Original Filed Dec. 9, 1940 2 Sheets-Sheet 2 lm ARC E X 77N6 U/SH/NG MATLR/AL WITH J5 B/NDfR 0F METHYL CELLULOSE i I 4'9 Y I ORGAN/C OR cam/we J0 INSULATION I 5/ l l l l ARC EXT/NGU/SH/NG MATERIAL W/TH BINDER a;

.' METHYL CfLLULOSf ARC E X T/NGU/S H/(VG MA TE R/A L W/ TH BINDER 0F METHYL CfLLl/LOSE .heatof an are but isfactory because of Patented Sept. 19, 1944 uumzo STATE METHOD OF 2,358,407 MAKING'ARO GONFINING STRUCTURES George F. McMahon,

Schweitzer 8; Conrad poration of Delaware Chicago, 111., assignor to Inc., Chica o, 11L, a cor- Original application December 9, 1940, Serial No 369,162, now

Patent No.

2,325,416, dated July 27, 1943. Divided and this application Novem- 3 Claims. (01. 1os-19'a) I her 6, 1942, Serial No.

My invention relates, generally, to circuit interruptingapparatus and it has particular relation to are extinguishing material used therefor and methods of making and moulding the same. This application is a division of my copending application, Serial No. 369,162, filed December 9, 1940, now Patent No. 2,325,416, issued July 27, 1943.

In electrical apparatus, particularly circuit interrupting devices, arc confining structures are provided around the path of the are that is formed on operation-thereof. The surface of these structures which is subjected to the direct heat of the arc is preferably formed of a material which will evolve an arc extinguishing medium that will sweep out the arc, deionize the arc space and prevent its restriking.

The material from which the arc extinguishing medium is evolved should be of high dielectric strength both before and after operation of the circuit interrupter. In some instances a material may be satisfactory from an insulating standpoint before it has been subjected to the afterwards it may be unsatthe formation of a conducting path over its surface as a result of carbonization of some of the material. This is known as tracking and is undesirable since a leakage path is provided over the surface of the are confining structure which may result in a dangerous condition in the circuit in which the circuit interrupting device is connected.

For many applications in electrical circuit interrupting devices, such as expulsion fuse tubes, sleeves for replaceable fuse links and plugs of insulating material used for are confining purposes, it is simpler and more economical to ex-' trude the arc extinguishing material into the desired shape. In so far as I am aware it has not been practicable in the past to extrude some of the more desirable arc extinguishing materials such as boric acid and magnesium borate.

Circuit interrupting devices in which are extinguishing materials are of doors in many instances where they are subjected to the weather. It is preferable that any extinguishing material which is used be impervious to water so that it will not swell, for example, and impair or prevent proper operation of the device. I

An object of this invention is to reduce the carbon content of the insulating structure surrounding an arc path in a new and improved manner.

Another object of this invention is to provide employed are used out for extruding arc extinguishing material into the desired shapes.

A further object of this invention is to provide water resistant arc extinguishing materials.

Other objects of this invention will, in part, be obvious and in part appear hereinafter.

For a more complete understanding of the nature and scope of this invention reference may be had to the following detailed description taken in connection with the accompanying drawings in which:

Figure 1 is a longitudinal sectional view of a circuit interrupting device having separable contact members in which the present invention is incorporated Figure 2 is a expulsion fuse device in which the present invention is incorporated;

Figure 3 is a view, in side elevation, of a replaceable fuse link, the insulating sleeve therefor being shown in longitudinal section;

Figure 4 is a view, partly in side elvation and partly in section, of a circuit interrupting device employing separable contact members and a plug of insulating materialformed in accordance with the present invention for confining and extinguishing the arc; and

Figure 5 is a view, partly in side elevation and partly .in section, showing an alternate forin of plug construction for the device shown in Figure 4.

According to this invention a relatively small amount of binding material, such as a cellulose ether is suitably combined with one or more different arcextinguishing materials to produce the desired shape for use in circuit interrupting apparatus. The binder is preferably methyl cellulose. Methyl cellulose is used on account of the ease with which it adapts itself to combinations with other materials.

Methyl cellulose is a water soluble cellulose ether that is obtained by methylating cellulose such as cotton. Solutions of methyl cellulose are not affected by changes in acidity or alkalinity over a relatively wide range.

The are extinguishing materials with which the binder of cellulose ether is employed are preferably those which will evolve water vapor and other gases when subjected to the heat of an arc. Certain inorganic materials are suitable for this purpose. Magnesium compounds such as magnesium borate, magnesium hydroxide, magnesium oxide and basic magnesium carbonate can be employed. Also, inorganic acids, such as boric acid, silicic acid, molybdic acid, and

longitudinal sectional view of an 1 the dry state before addi arsenicus acid. can be employed. Various comsary. The data thus far obtained indicates that it is unnecessary to employ more than 5% by weight of methyl cellulose as the binder when the foregoing inorganic arc extinguishing materials are employed. Usually, the amount of binder varies from 0.5% to about 4% by weight. Since a. relatively small amount of binder is employed, it will be obvious that the is negligible. Arc confining structures which are formed of one or more of arc extinguishing materials with a binder of methyl cellulose are substantiallyfree from tracking so .that their dielectric properties are substantially unimpaired to repeated severe arcing.

' In preparing the mix methyi cellulose is dissolved in water so as to produce a thick viscous liquid. The inorganic are extinguishing material is added to the-aqueous'solution of methyl cellulose in'powder form. PreferabLv the powder is fine enough to pass through a zoo-mesh sieve, although satisfactory results can be obtained if it is capable-of passing through only a 100 mesh sieve. The resulting'mixture is very thick and quite'rigid' and of such character that, when extruded or formed to the desired shape, it is capable of'retaining the same after the extruding apparatus has been removed so asto permit removal of the'extruded shape for drying and ing.

- Methyl cellulose is supplied commercially in I four diflerent viscosity grades in the form of white fibers, namely, low," medium, "high," and "extra high viscosity types. The low" grade'requires' more per unit volumeof water to make a thick viscous. solution than the "extra highgrade to make a solution of the same viscosity. In other words, when a solution of high methyl cellulose or solid. content is desired, the flow grade is usedand whena low solid content is desired the extra, high" grade is, used. This is important since the strength, shrinkage, etc., are partly controlled by adjusting this factor to shittheneeds.

- Standard methyl cellulose stock solutions can be made up as indicated in the following table,

dissolving the material in cold water or by adding hot water at thestart and then cold water.

,Solution No. Viscositytypc Grams c. f approx".

I per cent -1 Extrah 21.... 25 1,000 2.50 2..- do i i 12.5 1,000 1.25 .'i 10. 0 l, 000 1. 00'

Thedried, finely divided powder-is placed in a mixer and the correct volume of any' selected methyl cellulose solution added and the whole thoroughly kneaded and mixed. If more than one powder is used, the powders are mixedin g the methyl cellulose solution. Generally thefdough" v the mixture should be slightly more plastic when used to form liners inside tubing; and more rigid or solid if'formed into rods, "plugsfor similar solid bodies.

carbon content thereof even after being subjected the foregoing inorganic. I

It is desirable to keep the water content as low as possible at all times and at the same time still be able to form the object desired. This is to reduce shrinkage on drying out.

Methyl cellulose mixtures have the property of making an easy flowing plastic body when subjected to pressure without the water being squeezed out of the compound or plugging up the mold cavity or extrusion nozzle through separation of the solids from the water.

The following are examples of various mixes that can be made according to the present invention which are suitable for extruding purposes.

Example 1 70 grams of 100 mesh or finer magnesium hydroxide powder were added to 52 grams of a 5% -methyl cellulose solution, using the "low" viscosity grade. The whole was kneaded and mixed thoroughly and placed in an extruding or pressure tool and formed into rods or cylinders of approximately O. D. which were then baked at 60-80 C. and dried. The resulting product contained .about 3.5% methyl cellulose and the balance was magnesium hydroxide.

Example 2 v 53 grams of 100 mesh or finer magnesium bo rate powder were added to 34 grams of a 2.5%

extra high" viscosity methyl cellulose solution, thoroughly kneaded and mixed and used to make liners for insulating tubing such as a phenolic condensation product. The liners in this case were I. D. x .0. D. x 8%" long. No cracking appeared on'drying out or mild baking at 60-80 C. with or without a vacuumior several hours. The product was a good arc extinguishing material and a good insulator when tested at high voltages. This product when dry contains approximately 1.59% methyl cellulose and the balance magnesium borate.

Example 3 tubesthan those of Example 2 above, the ;liner- I .in this case being I. D. x "/s'f 0.13. x 4%" fl- Thedried tube. liner after baking at -70? C. contained approximately 3.7% methylpellulose and the balance magnesium borate and. magnesium hydrate in the following proportions:

. magnesium borate 83.0% and magnesium hyresulting from Cellulose.

drate 13.3%. During this process, tion of the magnesium oxide is converted into the hydrate because 01' the reaction of the water of the methyl cellulose solution.

- 25 grams of'magnesium oxide and grams of magnesium borate powders were thoroughly mixedand to this was added 44 grams of a 5% aqueous solution of "low"'viscosity grade methyl I The whole was thoroughly kneaded and-mixed and inserted in an-extrusiompress and used to form tube liners I. D. x 0.1) ii /i'long and also rods .617 O. D, x 256" long. On drying thetubeliners shrank zfl-in length No cracks appeared in either the tubes or rods.

the major per-- a synthetic plastic or ceramic I or porcelain, while thelinner layer or liner I2 is The final; composition. calculating to a dry :basis was asfollows: .magnesium borate 66.14%,

magnesium hydrate 36.18%, and methyl cellulose 11.94%. As inExampleB. the greater portion of the-magnesium-oxide is'converted into the hydrate.

" Example 5 111 grams of pure boric acid powder which passed through a 100 mesh sieve were mixed with 55 grams of a 2.5% extra his grade methyl cellulose aqueous solution. The whole was thoroughly kneaded and mixed and used to form small rods, about a" O. D. and tubing 1%" O. D.

with a 3 wall inside other insulating tubing. The methyl cellulose content on a dry basis was housing with the core therein is suitably held in extruding apparatus and the material for forming the liner is forced into the space between the that, when the circuit is interrupted under load conditions, an arc will be formed between the contact members it and I4. The heat of the arc will cause an arc extinguishing medium, 1. e., watervapor. to be evolved from the inner surface or the liner i2 which acts to deionize the arc space between the contact members is and M so as to prevent the are being restruck.

Referring now particularly to Figure 2 of the drawings, it will be noted that the reference character i5 designates, generally, an expulsion fuse tube which has an outer layer IQ of insulating material, such as a synthetic plastic body or a ceramic material, and an inner layer l1 that is formed of arc extinguishing material in the manner previously described. The outer layer i8 is provided with a plurality of peripheral grooves II in its inner surface into which the plastic mass forming. the inner layer i1 is extruded for holding the samein place.

At the upper end of the fuse tube i5 is a ter- 'minal [9 which is apertured so as to receive a core and the inner surface of the housing. After the space has been fliled. the core is withdrawn and the housing is placed in an oven where it is baked to drive off excess moisture.

Liners that are formed of magnesium borate with a binder of methyl cellulose are substantially free from cracks and have good mechanical and arc extinguishing properties. Because the amount of' methyl cellulose binder comprises a relatively small part of the liner, the amount of carbon that is depositedis negligible and consequently it is substantially free from tracking.

. Rods or plugs can be satisfactorily formed of magnesium hydroxide with a binder of methyl.

cellulose. When formed or extruded into this form they do not crack and when properly aged are highly resistant to water.

Tubes or liners can be formed usingvarying amounts of magnesium borate and magnesium,

.oxide with a binder of methyl cellulose. In this combination of materials magnesium hydrate is formed through the reaction between the oxide and the water used to carry the methyl cellulose.

. This reduces the'setting time of the plastic mass.

The resulting structure is somewhat stronger than if magnesium borate alone were used and it is less'likely to crack than is the case if magnesium oxide alone were used.

Referring now particularly to Figure 1 of the drawings, it will be observed that the reference character Ill'designates, generally, a housing or sleeve of insulating material for a circuit interrupter- The, housing I0 is formed by an outer layer H of suitableinsulating material suchas material, i. e., glass formed if arc extinguishing material combined with a'binder as described hereinbefore. The

26, formed preferably of silver, are secured to the lower end of the relatively infusible terminal 2i.

The other ends of the strain and fusible elements 25 and 26 are secured in the upper end of another relatively infusible terminal 21, to-which is secured a flexible conductor 28. The flexible conductor 28 extends out of the lower end of the expulsion fuse tube 26 and is connected to a second terminal 29-thereon by suitable clamp means SD.

When the liner I1 is formed of arc extinguishing material as disclosed herein, tests have shown that it is unnecessary to provide an additional sleeve of insulation for closely surrounding the .-strain and fusible elements 25 and 28 as is now generally considered necessary in the construction of universal fuse links. Even on relatively low current arcs, because of the relatively great availability of the water vapor in'the liner H, a suiflcient amount of the arc extinguishing medium-is evolved to sweep out the arc and deionize the arc space.

In Figure 3 of the drawings, there is shown generally at 33 a universal type link in which the present invention is incorporated. The fuse link 33 comprises a relatively infusible terminal 34 which is provided with a threaded stud portion 35 for receiving a flanged head 36. A strain element 31 and a fusible element 38. of material similar to the strain and fusible elements 25 and 26, respectively, of Figure 2, are connected to the lower end of the terminal 34 and to the upper end of another relatively infusible terminal 39. A flexible conductor 40 is secured to the lower end of the terminal 39 and it is provided with a special cone fitting 4| for adapting the fuse link 33 for use with certain types of fuse devices that are now well known to those skilled in the art. It will be noted that the flexible lead 40 extends beyond the fitting H for connection to a line terminal as may be desired.

Surrounding the strain and fusible elements 31 and. 30 is a sleeve, shown generally at 42, of insulating material. The sleeve 02 is formed by an outer layer 43 of material such as ilber and an inner layer 44 of arc extinguishing material that is formed as previously described using a binder of methyl cellulose. The layer ll of fiber is used for mechanical purposes and is preferably relatively thin, being only about ,6 inch thick. Other thinwalled plastic or ceramic bodies can be employed in lieu of ilber.

The terminal 34 is knurled, as indicated at 4|, to provideior frictionally gripping the composite sleeve 42v and holding it in place on the terminal 04 Figure 4 of the drawings there is shown a circuit'interrupting device which tormsa part of. a load break switch structure, the details of whichare i'ully disclosed in the copending application of Lindell et al., Serial No. 349,898,1iled August 2,, 1940. The load break device 48' is intended for use with a disconnecting switch and is employed for effecting the final circuit interruption. However, its uses are not limited to this particular application.

The switch 40 comprises astationary contact member ,40 that is generally ring-like in char-- acter and. it is arranged to be engaged by a rod-like contact member II that is disposed to be moved through a bore Ii that is provided by anextrueded plastic liner 82 which is formed as described .hereinbeiore using methyl cellulose as a binder for the arc extinguishing material. The liner 6! is held in place by a housing I; of suitable insulating material such as a synthetic plastic or a ceramic material irrwhich grooves 54 may be provided for the purpose indi-. cated hereinbeiore. The rod-like contact member 50 is arranged to be moved, out oi' and into contact engagement with the contact member I by means of an operating member il, only a portion of which is shown. Any suitable operating means can be provided for moving the rod-like contact member 5 0. r I

It will be understood that, when the rod-like contact member 50 is moved away from the stationary contact member 49 under load, an arc available in thei'orm of water vapor when they structure is'subiected to' the heat an electric are, which comprises, dissolving methyl cellulose in water in the proportion of about methyl cellulose to 95% water by weight, thereby producing'an aqueous solution of methyl celluloseymixing with said solution powdered boric acid in amount to iorm a homogeneous plastic mass which when subjected to pressure will flow readily while retaining its homogeneity, molding I the plastic mix under flowing pressure to the desired form and size of the structure to be produced, and drying the molded structure at a temperature to drive oi! excess moisture while retainins. nthe dried structure substantiallyall o! the combined water of the mix.

' 2. The method or making an arc extinguishing structure havingfa high water content rendered available in theiorm oi water vapor when the structure is subjected to the heat oi. an electric are, which comprises, dissolving methyl cellulose in water in the proportions of from about 2.5% to 5% methyl cellulose to from about 97.5% to 95% water by weight, thereby producing an aqueous solution oi methyl cellulose, mixing with said solution powdered boric acid'in amount to form a homogeneous plastic mass which when subiected to pressure will flow readily while retaining its homogeneity, molding the plastic mix under flowing pressure to the desired form and size oi the structure. to be produced, and drying the 1 molded structure at a temperature to drive oil will be formed within the bore 3| which should be promptly extinguished so as to quickly eifect circuit interruption with a minimum oi disturbance.

In order to effect this result an insert It is carried by and ismovable with the rod-like contact mem.

ber and it is moulded into a plug of arc extinguishing material having-a binder or methyl cellulose that is formed as previously described. That is, the p ug 51 is extruded around the in-: sert It. The plug 51 moves together with the rod-like contact member and "substantially illls the bore ll so that thearc which is drawn. between the contact members 40 and I0 is con-' fined between the outer surfacev oi the plug and the inner surface of the liner 5!. Under these circumstances'the arc is quickly blown out and the arc space is deionized so as to prevent restriking.

In Figure 5 of the drawings there is shown a plug ll which" can be used, if desired in lieu of theplug 51. It will be observed that the plug",

instead of having an insert extruded or moulded into the same, is drilled and tapped as indicated at'llior receiving a threaded stud. The plug I.

excess moisture while retaining in the dried structure substantially all of the combined water oi the mix.

a. The method of making an arc extinguishingstructure having a high water content rendered available in the form of water vapor when the structure is subjected to the heat of an electric arc, which comprises, dissolving methyl cellulose in water in the "proportions of from about 0.5% to 5.0% methyl cellulose to from about 09.5% to water by weight, thereby producing an squevoussolution oi methyl cellulose, mixing with said solution powdered boric acid or magnesium borate in amount to form a homogeneous plastic mass whichwhen subjected to pressure will flowreadily while retaining its homogeneity, molding the plasticmlx under flowing pressure-to the desired form and size of the structure to be produced, and drying the molded structure at a temperature to drive oil excess moisture while retaining in the dried structure substantially all of the combined water of the mix.

GEORGE r. McMAHON. 

